Stupid transponder question, or, stumped by the wife

So we were watching "Seconds From Disaster" on NGC last night, which
happened to be on AA 77. When they mentioned that seconds after gaining
control of the aircraft the hijackers turned off the transponder (along with
a brief explanation of what the transponder does), my wife simply says:

"That's stupid, why does something like that even have an on/off switch?"

I honestly never thought about it and couldn't answer her question. Does
anyone know?

Regards,

John K.

Can you picture the radar screen in Boston/New York/Chicago/LAX when all the
planes on the ground are showing up? The radar sweeps by and shows 23 planes
lines up for the departure runway, superimposed on whoever is flying
overhead.



"John Kirksey" wrote in message
news:_0YVe.24770$8h6.7135@trnddc09...
So we were watching "Seconds From Disaster" on NGC last night, which
happened to be on AA 77. When they mentioned that seconds after gaining
control of the aircraft the hijackers turned off the transponder (along
with
a brief explanation of what the transponder does), my wife simply says:

"That's stupid, why does something like that even have an on/off switch?"

I honestly never thought about it and couldn't answer her question. Does
anyone know?

Regards,

John K.

Or what Chicago center sees in east central Wisconsin the last week of
July, first week of August.....

Only 10.5 months until the next bash!

Steve Foley wrote:
Can you picture the radar screen in Boston/New York/Chicago/LAX when all the
planes on the ground are showing up? The radar sweeps by and shows 23 planes
lines up for the departure runway, superimposed on whoever is flying
overhead.

Irrelavant. The radar computer has software that will not show any
target within the user specified distance from the radar antenna. We
set ours to 1 mile. There are also altitude filters that can be set.
There is no good reason to ever turn your transponder off or to STBY
unless ATC asks you to or you somehow know it's not working properly.

"Newps" wrote in message
...

Irrelavant. The radar computer has software that will not show any target
within the user specified distance from the radar antenna. We set ours to
1 mile. There are also altitude filters that can be set.

Facilities that do radar approaches don't tend to filter out these targets.



There is no good reason to ever turn your transponder off or to STBY
unless ATC asks you to or you somehow know it's not working properly.


ASR on the field is good reason to turn your transponder off or to STBY
without ATC request.

Newps writes:

Can you picture the radar screen in Boston/New York/Chicago/LAX
when all the planes on the ground are showing up? The radar sweeps
by and shows 23 planes lines up for the departure runway,
superimposed on whoever is flying overhead.

Irrelavant. The radar computer has software that will not show any
target within the user specified distance from the radar antenna.
[...]

Maybe, but it is still a problem for aircraft with TCAS, who
receive false alarms due to the ground traffic.

- FChE

In article ,
(Frank Ch. Eigler) wrote:


Maybe, but it is still a problem for aircraft with TCAS, who
receive false alarms due to the ground traffic.

not if their radio altimeter is working.

--
Bob Noel
no one likes an educated mule

Frank Ch. Eigler wrote:

Newps writes:


Can you picture the radar screen in Boston/New York/Chicago/LAX
when all the planes on the ground are showing up? The radar sweeps
by and shows 23 planes lines up for the departure runway,
superimposed on whoever is flying overhead.


Irrelavant. The radar computer has software that will not show any
target within the user specified distance from the radar antenna.
[...]


Maybe, but it is still a problem for aircraft with TCAS, who
receive false alarms due to the ground traffic.


Then their TCAS has problems. Properly operating TCAS knows the
aircraft is stationary plus below a certain altitude it is advisory in
nature. If this were a problem I would hear about it from aircraft with
TCAS. Not once in the last 17 years has an aircraft with TCAS so much
as mentioned they were receiving an alert from an aircraft holding short
of the runway.

Newps wrote:

Irrelavant. The radar computer has software that will not show any
target within the user specified distance from the radar antenna. We
set ours to 1 mile. There are also altitude filters that can be set.
There is no good reason to ever turn your transponder off or to STBY
unless ATC asks you to or you somehow know it's not working properly.

There's still another problem called FRUIT (False Replies Unsynchronised
in Time). The radar bean has a width of about 3 degrees. All the
transponders in this small area respond with their code as soon as the
radar beam hits them. A major installation can handle up to ten codes
simultaneously. But if on a nice Sunday several hundred VFR air planes
are about the situation has arisen where the targets could no longer be
properly separated, with the result that the important echoes of
commercial traffic disappeared in the maze.

The controller can erase the VFR echoes, but when the radar con not read
them anymore the legibility of the remaining echoes is not improved.
Actually, this is one of the reasons why mode S is introduced.

Stefan

When diagnosing electrical problems, it's helpful to be able to
selectively turn things off.

When I'm downloading data from my EDM, I turn all the radio gear off to
avoid unnecessary battery drain.